Storm systems, driven by persistent low pressure near Alaska, are churning waters to dangerous levels along the West Coast of the United States.

The focus of the punishment from massive waves is from southern Washington state through central California. The San Francisco Bay area is one region taking the brunt, with waves expected to reach 30 to 40 feet (9 to 12 metres) in height through Tuesday.

An off duty NWS employee captured a large wave topping the Tillamook Rock lighthouse this PM. Large seas will continue well into Monday so please stay off jetties & use caution if you plan on visiting beaches along the Oregon and Washington coastline. #pdxtst#orwx#wawxpic.twitter.com/NWoVCxVcDs

This event is huge, and truly affecting the whole West Coast. High surf warnings extend from north of the Oregon and Washington border to about a two-hour drive north of Los Angeles.

High surf advisories or gale warnings stretch from the Canada to Mexico border.

“The dramatic wave heights are related to a consistent fetch of wind action that . . . has origins up near Alaska,” Marshall Shepherd, professor of atmospheric sciences at the University of Georgia, writes at Forbes.

(weatherbell.com modified by author)

As can be seen in the image above (where blue is low pressure and orange is high pressure), the cause of the widespread and life-threatening breakers along the West Coast is this massive upper-level low-pressure complex spinning around the Gulf of Alaska

The gigantic gyre is blasting storm after storm at the West Coast. It’s also fostering ideal conditions for strong winds to blow over hundreds of miles of ocean, virtually unaltered, which piles up mammoth waves in the process.

Very large extratropical cyclone in the northeastern Pacific, driving rain and mountain snow into Washington/Oregon. Notice the terminator (day/night line) fails to reach far north Alaska as we near the December solstice. Still prelim non-operational #GOES17 imagery past 24 hours pic.twitter.com/dJOCQIcOc9

Expectations are for conditions so extreme that the National Weather Service in the San Francisco Bay tweeted, “STAY WELL BACK FROM THE OCEAN OR RISK CERTAIN DEATH.”

The urgency highlights the unusual risk posed by this round of mega-swells.

Northern California, including the San Francisco Bay area, should end up with the worst of these pounding waves at the shoreline. The peak of the peak may be Monday, when waves in the 25- to 40-foot (8- to 12-metre) range could be common.